Barrel length has little to no affect on accuracy, your choice of bbs, barrel, and your hop up matter a lot more. Whatever accuracy increase you get from a longer barrel (if there is any increase), will not be worth the extra length. (You might actually see a decrease in accuracy since a part of the barrel will be "floating" in the silencer and could be more affected by vibrations)
My suggest would be:
Madbull or Prometheus barrel (Don't go for a barrel tighter than 6.03mm) Prometheus would be ideal. Madbull is supposed to be good for those on a budget.
Systema bucking
SCS (or an H nub, but they aren't as good)
Evike has all of the above.
BBs-I found G&G bbs to be of very good quality. (IMO better than KSCs) Goldenball bbs are also great, but they tend to leave a residue in the barrel and bucking.
Getting your gun to the 400 fps range will give you maybe an extra 10-20 feet of range. (it really isn't much of an increase at all) From 350+ fps with .2s to 380+ fps with .2s gave me about 10-20 feet.